[GSAS-II] GSAS-II Digest, Vol 291, Issue 1

[Toby, Brian H.]

The phase fraction for each phase multiplies the histogram scale factor and this product is proportional to the number of unit cells that are scattering. Thus the phase fraction values are on an arbitrary scale, as long as the histogram scale is allowed to compensate to match the [usually] arbitrary intensity scale.

To convert to weight fractions (or as preferred by NIST, mass fractions), one must consider the computed density for each phase and then normalize.
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From: GSAS-II <gsas-ii-bounces at aps.anl.gov> on behalf of LiaoNU via GSAS-II <gsas-ii at aps.anl.gov>
Sent: Friday, February 19, 2021 2:40 PM
To: Creuziger, Adam Abel (Fed) <adam.creuziger at nist.gov>; gsas-ii at aps.anl.gov <gsas-ii at aps.anl.gov>
Subject: Re: [GSAS-II] GSAS-II Digest, Vol 291, Issue 1

Adam,

Thank you and several others, especially Bob and Wenqian, who provided useful information. It turns out, to my best of understanding, the phase fraction refers to mole fraction, or number of unit cells. The weight fraction is then derived based on cell volume.

My data was collected at the APS at Argonne, which ran non-top off mode, meaning at certain times of a day the beam intensity deeply increases as a result of ring refill. The absolute value of phase fraction may be influenced, but the weight fraction, on the other hand, has some normalization calculation thus is not influenced.

Thank you all again.

Best,
Yifeng

On Fri, Feb 19, 2021 at 11:52 AM Creuziger, Adam Abel (Fed) via GSAS-II <gsas-ii at aps.anl.gov<mailto:gsas-ii at aps.anl.gov>> wrote:
Re Yifeng:

I am not sure on exactly what units are for "phase fraction”, but looking athletic GSASIIpwd.py it looks like this is the scale of the histogram for this phase. That can get a little convoluted with the Histogram Scale Factor in sample parameters.  You can set a constraint for this if you have multiple phases (constraints-> histogram/phase -> ‘Scale’   I end up with 0:0:Scale+1:0:Scale=1 as the equation)

The weight. fraction includes the atomic weight (including occupancy) divided by the unit cell volume. I found this description on line 132 of GSASIIddataGUI.py.

For exporting, Export-> Powder data as-> Text file  worked to some extent.  I’ve also been using the scripting toolkit, and that works well.

____________________________
Dr. Adam Creuziger
National Institute of Standards and Technology (NIST)
Gaithersburg, MD USA 20899.8553
Office 301-975-6015
adam.creuziger at nist.gov<mailto:adam.creuziger at nist.gov>

Visiting Research Affiliate Faculty, Metallurgical and Materials Engineering. Colorado School of Mines

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